A display device has been suggested which allows a real image (real mirror image) of an object being observed to be formed in the air so that an observer can view the image (see Patent Literature 1).
Such a display device includes a real mirror image forming optical system for allowing a real image (real mirror image) of an object being observed to be formed in a space toward an observer, and the object to be observed for which the real image (real mirror image) is formed and which is disposed in a space opposite to the space toward the observer of the real mirror image forming optical system. Such a real mirror image forming optical system allows a real image (real mirror image) of the object being observed to be formed at a symmetric position with respect to the symmetric surface of the real mirror image forming optical system.
Patent Literature 1 has suggested, as a real mirror image forming optical system or an optical element, a planar assembly of a plurality of unit optical elements (each being referred to as a dihedral corner reflector) that are each made up of two minute mirror surfaces (reflective surfaces) that are orthogonal to each other. Disclosed in Patent Literature 1 is a dihedral corner reflector array optical element in which a plurality of dihedral corner reflectors are disposed in an array on one plane, that is, aligned in a grid pattern. Some dihedral corner reflector array optical elements employ a dihedral corner reflector that utilizes, as a mirror surface, the inner wall of an optical through-hole that assumes a direction in which the through-hole penetrates the element plane; or a dihedral corner reflector that utilizes, as a mirror surface, a transparent cylindrical inner wall that is protruded from a surface of a substrate made of a transparent material (see Patent Literature 2). Note that since the transparent cylindrical body is a prism, such as one having a prismatic shape, in which an optical hole is cylindrically projected and a translucent material is filled therein, the transparent cylindrical body will be hereinafter referred simply to as the projection.
The dihedral corner reflector array optical element is configured such that each mirror surface of each of a plurality of aligned dihedral corner reflectors is erected to be generally perpendicular to the element plane. Thus, a light beam from an object being observed that is disposed toward one side of the element plane is reflected twice on the dihedral corner reflector when passing through the element plane, and the light beam is bent to pass through the element plane so as to be formed as a real image in a space toward the opposite side of the element plane and in which the object being observed is not present. That is, the dihedral corner reflector array optical element is capable of forming the real image of the object being observed as if the object being observed is present at a symmetric position with respect to the element plane (also referred to as a symmetry plane) of the dihedral corner reflector array optical element.